Propane Tank Installation Information
When you purchase a propane tank holder, it will come with the strap already installed. The best thing is to use it the way it arrived. If you have removed the strap, no problem. The trick is to run the strap right back from the way you threaded it to make it tight. These images are likely more helpful than the words.
You want to end up with the tank being snug as a bug in a rug.